Results of family, twin and adoption studies indicate that genetics is of etiological important in Bipolar Affective Disorder (BPAD). Investigations show that there is considerable family aggregation for bipolar illness. In these studies there is not only an excess of illness in the relatives of probands, but there is increased risk correlated with relatedness. Genetic research has resulted in promising leads for the location of disease-causing alleles. More recent molecular genetic techniques hold promise for discovery of the genetic mechanism(s) for bipolar illness. Recent discoveries of unstable DNA (aberrant expansion of exonic trinucleotide repeats) in the inherited neuropsychiatric disorders of Fragile X syndrome, myotonic dystrophy and Kennedy's disease have generated interest in whether expanding trinucleotide repeat sequences (TNRs) are related to psychiatric disorders, particularly schizophrenia and bipolar illness. This novel mechanism may have relevance because these psychiatric disorders exhibit two features which are common among the neuropsychiatric disorders with unstable DNA:nonmendelian inheritance patterns and genetic anticipation. This research tests the unstable DNA hypothesis of bipolar affective disorder (BPAD). The hypothesis to be tested is that there will be a significant shift toward larger products in probands than controls.